6S T. C. WHITE ON THE HISTOLOGICAL DEVELOPMENT 



or how they took place it is not possible to state, so gradual was 

 the process. The larva under observation, and which upon 

 several occasions I had the pleasure of submitting to your notice, 

 monopolised the stage of my microscope for one month, when it died, 

 but you may see by looking at others collected at the same time as 

 this the changes which have been produced in that time. From the 

 rudimentary condition I have described it has passed into a stage 

 wherein its internal anatomy is considerably advanced, and it is pre- 

 paring by other interstitial changes for assuming its pupal existence. 

 I cannot give you the definite times at which these changes took 

 place, but can only note them somewhat in the order in which they 

 occurred, taking first the various segments and their special con- 

 tents, and then those parts which are common to the whole body. 

 The first segment or head, with its pairs of formidable jaws, 

 naturally claims our primary attention. Professor Ray Lankester, 

 at the time his father occupied this presidential chair in 1865, wrote 

 a short but accurate account of the anatomy of this larva, which 

 was published in the " Popular Science Review" for that year, and 

 which I should recommend to your notice. In this article he 

 divides these jaws thus : — The first of these pairs he terms Tarso- 

 gnaths, or oar-like appendages ; each consists of two parts, a stem 

 and four long terminal bristles. The next pair are termed Tricho- 

 gnaths, or labial bristles. The third pair are very curiously serrated, 

 and called Pristognaths, or saw-like organs. Then next in order, 

 counting from the end of the beak, comes a central prehensile organ. 

 It is a projecting, cylindrical body, capable of being moved back- 

 wards and forwards, having its top crowned by two groups of hairs. 

 This is called the Mesognath. This can be used either as a finger 

 and thumb by the apposition of the two lips carrying the respective 

 groups of hairs with which it is crowned, or it can be used as a 

 broom to sweep prey into its mouth. When bent forward it falls 

 easily and naturally between the two jaws, which are placed next in 

 order, and called Platygnaths, very powerfully constructed, and 

 capable of crushing the unfortunate victim who once gets entangled 

 in their grip. All these organs are furnished with strong muscles, 

 and these were, as you will suppose, more developed than those 

 supplying the rest of the body, and not so important to the creature's 

 subsistence, for as Corethra, as we will call this larva for brevity's 

 sake, is reckoned a very predacious and voracious subject, and as 

 development presupposes and requires food to keep it up, so here 



